support loading fit checkpoints in JacobianLens.load()

[priyanka25aug]

What this does

Extends JacobianLens.load() to accept fit checkpoints (files saved by the fitting pipeline with a jacobian_sum key) in addition to the existing artifact format (files with a J key).

The conversion lives in a new _from_checkpoint_payload() classmethod:

• Divides the running Jacobian sums by n_prompts to recover the per-prompt mean
• Harvests safe scalar provenance keys (model_name, model_revision, corpus) from the flat payload namespace into metadata
• Strips fit-reserved keys (transformer_lens_fit, transformer_lens_version, model_system, etc.) so they don't leak into the converted lens
• Drops tensor-valued metadata fields that can't survive weights_only=True reload, recording their names and shapes in dropped_fields for transparency
• Sets converted_from: "jacobian_lens_checkpoint" so merge() naturally refuses to mix converted and natively TL-fitted lenses (provenance keys differ)

Raises ValueError with a clear message if the file has neither key, or if n_prompts <= 0. Tuned-lens support deferred.

Files changed

• transformer_lens/tools/analysis/jacobian_lens.py — new _FIT_RESERVED_KEYS and _CHECKPOINT_FLAT_PROVENANCE frozensets; updated load(); new _from_checkpoint_payload()
• tests/unit/tools/test_jacobian_lens_import.py — unit tests on synthetic fixtures covering artifact regression, checkpoint round-trip, dtype preservation, n_prompts=0 guard, tensor field dropping, fit-key stripping, and merge provenance rejection; no model/oracle dependency
• docs/source/content/jacobian_lens_fitting.md — "Importing an existing lens" section added

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have not rewritten tests relating to key interfaces which would affect backward compatibility

Testing

uv run pytest tests/unit/tools/test_jacobian_lens_import.py -v

[priyanka25aug]

Hi @jlarson4 — all format, type, docstring, and benchmark checks are passing. The remaining long-running jobs (compatibility across Python 3.10/3.11/3.12 and full coverage) are still in progress but looking clean so far.

Would you mind taking a look when you get a chance? Happy to make any changes based on your feedback.

[jlarson4]

Hi @priyanka25aug! Thanks for taking this on. Checkpoint import is a genuinely useful capability, and the conversion math, artifact-path safety, and merge sentinel are all done right.

A couple comments below that we should address before merging

[jlarson4]

The reference package's write_checkpoint() stores the count as n_done, so genuine checkpoints are rejected here with n_prompts=0. Could the loader read the key set that writer actually emits, as the pre-existing fixture does (tests/unit/tools/test_jacobian_lens.py:507)?

[jlarson4]

Real checkpoints can record a non-final target_layer, but this conversion discards the key, so the target-convention refusal in validate_model silently passes (jacobian_lens.py:605). Could the conversion preserve target_layer, or refuse checkpoints recording a non-final target?

[jlarson4]

These fixtures encode the loader's own invented schema, so the suite continues to pass while real reference checkpoints fail to load. Once the schema is fixed, could one fixture mirror the reference write_checkpoint() payload key-for-key so format drift causes a failure?

[jlarson4]

Tuned-lens translators are affine and carry the bias. It is the Jacobian artifact format that has no bias slot to receive it. Can we ensure this detail is included as part of this note?